University of Chicago researchers have found that women prefer the scent of some men over other men because of genes they have inherited from their fathers. This is the first time scientists have demonstrated that people can actually inherit preferences, said Martha K. McClintock, the David Lee Shillinglaw Distinguished Service Professor in Psychology at the University, and the co-author of a paper that will be published in the February edition of the journal Nature Genetics.

The research also shows that women prefer the odor of males to whom they are genetically similar, but not identical, over those that are either nearly identical or completely unfamiliar.

"This finding provides a mechanism for understanding, in part, the biological basis for several human social behaviors, possibly including relationships with siblings, friends and even mate selection," said co-author Carole Ober, professor of Human Genetics at the University of Chicago.

McClintock, who is one of the world's leading experts on pheromones and chemosignals, said the findings demonstrate the extraordinary sensitivity of the olfactory system. "Women can actually smell genetic differences," she said, "They can smell differences as small as a single gene."

In order to conduct their experiment, the researchers had to determine each study participant's HLA (human leukocyte antigen) gene sequence, the inherited basis for personal odors and olfactory preferences. HLA genes play a key role in each person's immune system, enabling the body to distinguish between cells that are part of the body and potential threats, such as bacteria, or parasites. HLA genes vary from person to person; millions of possible combinations exist, and only identical twins have matching HLA types.

Suma Jacob, a University of Chicago postdoctoral fellow who conducted the study, compared the women's responses to a variety of mild odors, both human and non-human. The non-human fragrances were household odors, such as bleach, clove, cotton and cardboard.

The human odors were gathered using the "t-shirt method." Men, who were selected because of their HLA types and diverse ethnicity, wore t-shirts for two consecutive nights to absorb a mild odor. The women, unaware of whether they smelled non-human or human scents, were then presented with 10 boxes, each of which included a scent. They were asked to rate each scent on four key attributes: familiarity, intensity, pleasantness and spiciness.

"Our goal was not to measure which scent women were sexually attracted to," McClintock emphasized. "Rather, our goal was to find out what smell these women wanted to be around all the time." For instance, she added, "you may love the smell of garlic, but not want to be around it 24 hours a day."

After completing the "odor test," the researchers compared the HLA sequences of the women, and the male odor donors they preferred. "A clear pattern emerged," said Ober. "The women did not choose the scents of men with genes totally similar to their own or totally dissimilar to their own. They chose men with an intermediate level of difference."

This finding, Ober said, could answer an important question about mating choice. In 1997, Ober and colleagues found that young people from an isolated ethnically homogeneous population somehow managed to avoid choosing spouses that were genetically too similar to themselves, without any way of knowing each other's genetic make-up. Marrying someone too similar increases a couple's risk for miscarriage or for passing on recessive genetic disorders.

"We had convincing data that genes in the HLA region could influence this important social choice, but until now we had no persuasive explanation for how these genes might exert this influence," Ober said.

The researchers also determined that a woman's preference for male odor choices was paternally inherited. By looking at the genes of the women's parents, McClintock said, "we found that a woman's odor choice is based on HLA alleles inherited from her father – but not her mother."

The research suggests a romantic notion about mating choice, McClintock said. "Our research indicates that there is not one most preferred male odor for everyone, but that odor preference is relative," McClintock said. "It's thoroughly unique and based on the degree of HLA differences between a man and woman."